1. Field of the Invention
The present invention relates to a silicon nitride composite sintered body having high strength and high toughness and a process for producing the composite sintered body, and provides structural ceramic materials to be used for automobile parts, wear-resistant tools, etc.
2. Description of Prior Art
Silicon nitride is a material well-balanced in strength, fracture toughness, and resistances to corrosion, abrasion, thermal impact, oxidation, etc., and has attracted attention recently as an engineering ceramic for structural materials to be used at room temperature and further at elevated temperature. However, in order to put silicon nitride ceramics to use in the field where high reliability and safety are required of the materials of construction such as automobile parts, it is indispensable to further improve the fracture toughness of the ceramics to overcome the brittleness thereof and also to contrive an increase in the strength thereof. There is disclosed, for example, in Japanese Patent Publication No. 265173/1987 a technique for improving the fracture toughness of a ceramic material by combining silicon nitride matrix with silicon carbide whisker to disperse the whisker in the matrix. It is believed that this method can achieve the improvement in the fracture toughness of a ceramic material because the crack which progresses and expands during the fracture is deflected by the whisker, or extraction or crosslinking of the whisker takes place. Although the fracture toughness of the ceramic material is improved by the combination thereof with the whisker, it is substantially difficult to completely remove the agglomerate of the whiskers by mechanical means since the size thereof is of the order of 1 to 10 .mu.m and thus the agglomerate as the coarse grain functions as a breaking point, thereby decreasing the strength of the ceramic material.
According to the conventional methods, therefore, it has been difficult to simultaneously improve the strength and the toughness of silicon nitride ceramics, since the fracture toughness thereof has been improved by the addition of the whisker or by allowing large columnar crystals to exist by grain growth of silicon nitride, which has resulted in substantial expansion of the size of defects and a decrease in the strength thereof. Accordingly, it has been a serious problem how to reconcile the improvement in the strength with that in the toughness of a silicon nitride ceramic having a structure in which the silicon nitride matrix is constituted of fine uniform particles free from coarse grains.